In vacuum processing in general, and in processing of substrates on which electronic elements are being formed in particular, the speed with which vacuum pressure is achieved in a vacuum chamber is a critical path step in achieving specified vacuum process conditions so that substrates can be cycled and processed. Similarly, once stable substrate processing conditions are achieved, maintaining the integrity of the low pressure process requires that the process environment remain stable by minimizing pressure transients both short and long term which may cause or contribute to anomalies in process conditions.
The current state of the art is such that short fat substrate vacuum processing chambers are machined from a solid block of the base material. As a result, as much as 80% of the material is cut away before the final chamber configuration is arrived at. Similarly in cluster arrangements of processing chambers, the cluster tool housing is carved (machined) from a large block of the base material, again, much of it is wasted as it is cut away. The base material, usually a cast piece is specified to include a high degree of quality (material integrity) to assure that there are no openings through which gasses may leak into the chambers.
When tall narrow vacuum chambers are needed, the technique of machining material away from a block of base material is abandon in favor of using large thick plates. Such plates are bent 90.degree. to form an L-shaped cross section. An example of L-shaped cross section plates is shown in FIG. 1 (e.g., 22). Four of these L-shaped cross sections are welded together at their edges using butt welding techniques to form a tube which acts as the wall of the processing chamber. An example of the construction of this type of prior art arrangement is shown in FIG. 1. A set of four corner panels 22, 24, 26, and 28, having been bent into L-shaped pieces are complimented by a top plate 32 and a bottom plate 34. The location of a future substrate access opening is shown by dashed lines 30. The whole configuration of elements defines a process chamber assembly 20, which in this case is a relatively tall and thin load lock module as would be used associated with a cluster tool.
Once fixed elements of the process chamber assembly are joined by welding the configuration as seen in FIG. 2 is formed. The four corner panels 22, 24, 26, 28 having been welded together by four butt weld type joints whose weld region and heat affected zone are depicted by weld regions 52, 54, 56, and 58. Welding is done inside and out. Since the only performance criteria for the welds is pressure sealing, grinding and multipass welding is not uniformly performed, as is done in welds where the highest degree of quality assurance is required by specification. The bottom plate 34 is welded to the bottom of the tube formed by the welded plates, The weld region and heat affected zone of the bottom weld is represented by the lower weld area (heat affected zone) 60. Again the welding is done both inside and out, the inside weld being difficult to perform as it is being done at the bottom end extreme corner of a narrow closed end tube. Both the sidewall and bottom plate welds are minimally inspected and often contain inclusion, voids, and porosity.
Similarly the corners of the sidewall plates where they were bent contain, surface defects which are created or accentuated by the bending of the thick plate (for example--3/4 inches (19.05 mm) thick).
As a result of the side and bottom plates being welded their final dimensions are uncertain, for example an O-ring groove 44 (shown in FIG. 3) in the end surface of the tube assembly, is intended to be centered in the end surface, with a particular configuration. Because the locations (dimensions) of the inside and outside surfaces vary slightly depending on how the welding was completed, before machining a rectangularly configured O-ring groove in the end surface of the tube, a machinist or machine tool must go through measuring steps to find a datum line between the inside and outside surfaces and then calculate where the machining should take place so that the O-ring groove when completed does not approach either an inner or an outer surface too closely.
An end view of the processing chamber of FIG. 2 is shown in FIG. 3. The locations of heat affected, weld zones are shown on each side (e.g., 42). The O-ring groove 44 is cut in the end surface to mate with the removable (top) cover plate.
A parameter specified in the material specification for a finished product is surface finish. Once all welding and machining is complete, a specified surface finish must be achieved on the surfaces of the plates both inside and out. Often the treatment of the surfaces to achieve a specified surface finish is a hand operation which is time consuming and variable. Inspection of such finishes is also subject to high rate of rejection for refinishing. It would be preferred to reduce the variability, improve uniformity, and reduce the time associated with achieving an acceptable surface finish/ Conventional vacuum chamber vacuum standards, find both machined and welded vacuum chamber structures acceptable when they are deemed tight, in that a vacuum pressure of 10.sup.-9 torr can be reached quickly and vacuum pressures of 10.sup.-5 to 10.sup.-8 torr can be maintained during processing. Improvement in vacuum performance are desired as a typical pumpdown time for a system is 20% (at the initial stage and further pump down required every time a wafer is being loaded and unloaded). A reduction in the pump down time can significantly affect product throughput.